The present invention relates to a motor-driven type power assisted steering control device for an automobile. More particularly, it relates to such apparatus of a type that operations of a steering wheel are assisted by a motor on the basis of signals of a steering torque sensor.
A conventional motor-driven type power assisted steering control apparatus is so constructed that a torque sensor detects a torque of steering; an output from the torque sensor is supplied to a control means so that an auxiliary torque in proportion to the output of the torque sensor is generated by means of a motor, and the auxiliary torque is given to a steering shaft to thereby assist a power for operating the steering wheel.
FIG. 2 is a diagram of a control circuit for a conventional motor-driven power assisted steering control apparatus wherein a reference numeral 1 designates a torque sensor, a numeral 16 a motor control circuit, numerals 15A, 15B input terminals and a numeral 2 an interface circuit for a steering torque detection signal, through which the steering torque detection signal is supplied to a microcomputer 3 (hereinbelow, referred to as CPU). A numeral 4 designates an interface circuit for supplying a signal to turn a motor in the right direction, through which a signal for driving the motor in the right direction is input from the CPU 3 to a motor driving circuit 10. A numeral 5 designates an interface circuit for supplying a signal to turn the motor in the left direction, through which a signal for driving the motor in the left direction is inputted from the CPU 3 to the motor driving circuit 10. Numerals 13A, 13B respectively designate output terminals through which terminals of the motor 14 are connected. A numeral 6 designates a D/A conversion circuit. An error amplifier for controlling the motor 14 has an input terminal which receives a torque detection signal from the CPU 3, which is inputted as an analog signal after having been subjected to D/A conversion, and another input terminal which receives an output from a detection circuit 11 for detecting a motor current. A numeral 9 designates a pulse width modulation signal oscillator which produces a signal for pulse width modulation (PWM) control, and the output of the oscillator 9 is inputted to a PWM modulator 8 along with an output from the error amplifier 7. The PWM modulator 8 supplies a PWM signal to the motor driving circuit 10. A numeral 12 designates a resistor for detecting a motor current.
FIG. 3 is a diagram showing the output characteristics of the torque sensor 1 wherein the abscissa represents a steering torque in the right or left direction and the ordinate represents a torque output. A symbol T.sub.0 indicates a neutral point of steering torques (the zero point of torque). The right region of T.sub.0 represents a steering torque for a steering wheel turned in the right direction and the left region represents a steering torque for the steering wheel turned in the left direction. A symbol T.sub.2 represents a steering torque in which a control in the right direction is started and a symbol T.sub.1 represents a steering torque in which a control in the left direction is started. Symbols V.sub.0 -V.sub.2 respectively designate detection outputs corresponding to T.sub.0 -T.sub.2. The output characteristics are substantially linear in a range of control. As a result, the region between T.sub.1 and T.sub.2 constitutes a non-sensitive zone of control.
FIG. 4 is a diagram showing the output characteristics of the motor 14 wherein the symbols V.sub.1 -V.sub.2 are the same as defined above, V.sub.3 designates a saturated torque detection output in the right direction and a symbol V.sub.4 designates a saturated torque detection output in the left direction. The motor output changes in a substantially linear fashion between V.sub.2 and V.sub.3, and it changes in a substantially linear fashion between V.sub.1 and V.sub.4 so as to be symmetric therewith. The motor output becomes zero between V.sub.1 and V.sub.2 and the motor output exhibits a constant value P.sub.max in the region greater than V.sub.3 or smaller then V.sub.4.
The operation of the conventional device a shown in FIG. 2 will be described. When an operator handles a steering wheel, a torque detection signal in proportion to a steering torque of the steering wheel is outputted from the torque sensor 1, and the signal is inputted to the CPU 3 through the interface circuit 2. The CPU 3 outputs a torque detection signal as a digital signal. The torque detection signal is converted into an analog signal by means of the D/A conversion circuit 6. The direction or polarity of the torque detection signal is determined in the CPU 3. As a result, a driving signal for driving the motor in the right direction is supplied to the interface circuit 4 or a driving signal for driving the motor in the left direction is supplied to the interface circuit 5. The motor driving circuit 10 receives a driving signal from the interface circuit 4 or 5 to drive the steering wheel in the right or left direction; thus an instruction of turning direction of the motor 14 is provided.
The output of the D/A conversion circuit 6 is inputted to the error amplifier 7, and the output of the error amplifier 7 is inputted to the PWM modulator 8. The output of the PWM modulation oscillator 9 is inputted to the PWM modulator 8. As a result, a control signal having a pulse width in proportion to the torque detection signal is supplied from the PWM modulator 8 to the motor driving circuit 10 so that the torque of the motor 14 is controlled on the basis of the output of the motor driving circuit 10. A current flowing in the motor 14 is detected by the motor current detection circuit 11, whereby the motor current is limited or is interrupted depending on a detected level of the current.
However, when a locking phenomenon or the like takes place in the motor 14 in the conventional apparatus, the steering wheel can also become locked or become extremely heavy and difficult to handle, which causes a dangerous state.